Bacillus thuringiensis
Bacillus thuringiensis ("Bt") is a gram-positive soil bacterium, which produces endogenous crystalline inclusions upon sporulation. Early in this century, these bacteria were found to be insecticidal (Berliner, 1915). Some years later, their insecticidal activity was found to reside in the proteins present in their crystals, hereinafter referred to as "insecticidal crystal proteins" or "ICPs". Since then, the Bt strains, spores, crystals and ICPs have been used as biological insecticides in commercial formulations.
The limited spectrum of these insecticidal proteins allows any naturally occurring predators of the target insects to survive. The continued presence of these predators prevents further outbreaks of the insects. Furthermore, these Bt proteins have the advantage that they are rapidly degradable and that no stable residues accumulate in the environment.
Cry proteins and cry genes
The specificity of the environmentally safe Bt insecticides has provoked a search for new Bt strains, producing toxins against other insect pests. Insecticidal Bt strains toxic to lepidopteran, coleopteran and dipteran insects have been found (Hofte and Whiteley, 1989). Although considerable homology can be found between genes that encode various ICPs toxic to one particular insect class, the sensitivity of specific insects to related Bt gene products is often very different. For instance, Chambers et al (1991) described a large difference in activity of the CryIF protein against Heliothis virescens and Heliothis zea (50% lethal concentrations of respectively 0.31 and &gt;57 ng protoxin/mm.sup.2 diet).
The Bt insecticidal crystal (Cry) proteins have been divided into five classes, according to their structural similarities and insecticidal spectra (Hofte and Whiteley, 1989): CryI proteins are toxic to Lepidoptera, CryII proteins are toxic to Diptera and Lepidoptera, CryIII proteins are toxic to Coleoptera and CryIV proteins are toxic to Diptera. A general cytolytic protein (cytA) is classified as a fifth toxic protein, but it has no specific insecticidal activity. The Bt genes coding for the insecticidal Cry proteins (cry genes) show strong homology in some conserved domains. These insecticidal Bt genes are mostly found on large conjugative plasmids, which may explain their observed mobility among Bt strains. One strain can contain several cry genes, and one gene can be found in several strains (Hofte and Whiteley, 1989).
Typically, cryI genes encode proteins with a molecular weight of 130 to 140 kD (hereinafter referred to as the "protoxins"), and upon ingestion by a sensitive insect, the protoxins are processed to smaller proteins (hereinafter referred to as the "toxins") having a molecular weight of 60 to 70 kD. The cryII and cryIII genes encode protoxins with a molecular weight of about 70 kD (except the cryIIIC gene which encodes a protoxin of 129 kD according to PCT publication WO 90/09445). The CryIV genes encode protoxins of either of these molecular weight types. The CryI protoxins constitute the largest group of protoxins, which are found in typical bipyramidal crystals.
The cry genes have been used to transform bacteria (e.g., Obukowicz et al, 1986; Stock et al, 1990) and plants (e.g., Vaeck et al, 1987) in order to provide resistance against insect pests. Adequate expression in plants was only obtained when the plants were transformed with a truncated Bt gene (e.g., Vaeck et al, 1987; Fischhoff et al, 1987; Barton et al, 1987).
The cryIB gene has been described in European patent publication ("EP") 408 403 and by Brizzard and Whiteley (1988). It encodes a 137 kD protoxin and a 66 kD toxin. The CryIB toxin has been shown to be insecticidal to insects like Pieris brassicae, Plutella xylostella, Spadoptera littoralis and Spodoptera exigua (Ferre et al, 1991; Visser et al,
The CryIAa (Gawron-Burke and Baum, 1991), CryIAb, CryIAc (Macintosh et al, 1990) and CryIF gene products (Chambers et al, 1991) have been described as toxic to O. nubilalis. Moreover, Peferoen (1991) has described the insecticidal activity of the following ICPs against various insects, including O. nubilalis: CryIAa, CryIAb, CryIAc, CryIB, CryID, CryIC and CryIE, and PCT publication WO 92/09696 also has described the insecticidal activity of the cryIAb and cryIB genes against O. nubilalis.
PCT publication 90/15139 has described the prevention of insect resistance development with various combinations of Bt genes, such as the cryIAb and cryIB genes (the Bt 2 and Bt 14 genes), against Pieris brassicae, Plutella xylostella, and Phthorimaea operculella.
Mode of action of the CryI proteins
The ICPs owe their specificity to the presence of specific receptor sites in the midgut brush border membranes of sensitive insects. In vivo, the crystals are solubilized in the alkaline environment of the midgut, and the released protoxins are processed by proteases to yield smaller protease-resistant toxins which bind to, and cause swelling of, the midgut cells (Gill et al, 1992). The C-terminal part of the CryI-type protoxin is probably involved in the formation of its crystal structure, but is thought not to be important in its mode of action (Hofte and Whiteley, 1989). Electrophysiological evidence (Harvey et al, 1983) and biochemical evidence (Knowles and Ellar, 1987) suggest that the toxins generate pores in the midgut brush border cell membranes, thus disturbing the osmotic balance. The intoxicated insects quickly stop feeding and eventually die. The high affinity binding of the toxins has been correlated with their toxicity (Van Rie et al, 1989).
Ostrinia nubilalis
The European corn borer is a very serious and persistant pest for corn (Davidson and Lyon, 1987; Hudon et al, 1987). The larvae of this insect initially feed on leaf tissue and later enter the stalks, burrowing downwards as the season progresses. O. nubilalis is estimated to be the most important corn pest in Europe and the second most important in the USA. Damage caused by O. nubilalis in the USA is estimated to be over 400 million dollars (U.S.) a year. Estimates for O. nubilalis spraying amount in France to 25 million dollars (U.S.) a year. Up to now, hazardous chemical insecticides have mostly been used to combat this insect. The European corn borer is remarkably polyphagous (Hudon et al, 1987) and has been found to attack other important crops such as wheat, cotton, potato, tomato, beet, oat and soybean plants (Davidson and Lyon, 1987; Hudon et al, 1987).